The present invention relates to a polyester continuous production process. More specifically, it relates to a process for producing a polyester comprising an aromatic carboxylic acid and an alkylene glycol having 2 to 4 carbon atoms as main components stably and continuously while low-boiling substances are removed from a glycol component recovered from a polycondensation reaction step to recycle the glycol component.
Heretofore, polyesters comprising an alkylene glycol having 2 to 4 carbon atoms as the main glycol component, especially polyalkylene terephthalate has been widely used for various purposes due to its excellent physical and chemical properties. Particularly, fiber, film and other molded products thereof are widely used in industrial textiles such as clothing and tire cords, engineering plastics and the like thanks to their excellent mechanical properties such as strength and elastic modulus, and heat resistance.
Generally speaking, polyalkylene terephthalate for use in the above application fields is produced by a direct polymerization method or ester exchange method. The direct polymerization method comprises the steps of forming a polyester precursor through a direct esterification reaction between an acid component and a glycol component and polycondensing the polyester precursor at normal pressure or reduced pressure. The ester exchange method comprises the steps of forming a polyester precursor through an ester exchange reaction between a lower alkyl ester of an acid component and a glycol and polycondensing the polyester precursor at normal pressure or reduced pressure.
The glycol component distilling out from the polycondensation reaction step is generally condensed by a tube type heat exchanger or wet type condenser, recovered and re-used as part of a raw material. Generally speaking, the distilling alkylene glycol having 2 to 4 carbon atoms contains various low-boiling substances by-produced mainly by a decomposition reaction which occurs during a polycondensation reaction and by other side reactions as well as water by-produced in the esterification reaction step and a lower alkyl alcohol by-produced in the ester exchange reaction step. For example, in the production of polybutylene terephthalate, 1,4-butanediol distilling out from a polycondensation reaction contains tetrahydrofuran, water and the like formed by the dehydration/cyclization reaction of 1,4-butanediol itself. In the production of polyethylene terephthalate, ethylene glycol distilling out from a polycondensation reaction contains acetaldehyde, 2-methyl-1,3-dioxolan, methyl cellosolve, 1,4-dioxane, water and the like formed mainly by a decomposition reaction which occurs during a polycondensation reaction. In the production of polypropylene terephthalate, 1,3-propanediol distilling out from a polycondensation reaction contains acrolein, allyl alcohol, 3-ethoxy-1-propanol, water and the like formed mainly by a decomposition reaction which occurs during a polycondensation reaction. Therefore, when the above glycol component is directly used as a raw material without being distilled, the distillation load of a distillation column attached to an ester exchange reactor or esterification reactor is increased or changed by these low-boiling substances contained in the glycol component, thereby causing such a problem that the production process becomes instable. Particularly when the amount of water contained is large, the activity of a reaction catalyst is impeded, the reaction factor does not become constant, the production process becomes instable, the quality of the final product varies, or the color of the final product worsens. To avoid these problems, it is desired that the glycol component should be subjected to a distillation step separate from a system to be purified before it is recycled. Since this method requires not only a large amount of investment in the installation of equipment such as a distillation column and a storage tank but also high running cost, it cannot be said that this method is economically advantageous. Therefore, there are proposed processes which the process does not become instable even when the distilling glycol component is recycled as a raw material without purification.
As one of the processes, JP-B 55-33734 (the term xe2x80x9cJP-Bxe2x80x9d as used herein means an xe2x80x9cexamined Japanese patent publicationxe2x80x9d) discloses a polybutylene terephthalate production process in which a glycol component distilling out at a pressure of less than 101 kPa and 0.27 kPa or more in-a polycondensation reaction step as a distillate from the polycondensation reaction step is recycled as a raw material without purification by a distillation step. JP-A 10-279677 (the term xe2x80x9cJP-Axe2x80x9d as used herein means an xe2x80x9cunexamined published Japanese patent applicationxe2x80x9d) discloses a polyester direct polymerization method in which a glycol component distilling out from a second esterification reaction tank is recycled as a raw material without purification by a distillation step. JP-A 09-124783 discloses a polyester production process in which a glycol component distilling out from a polycondensation reaction step is recycled as a raw material to be added to an esterification reaction tank in a method of direct polymerization without purification by a distillation step. However, as the glycol component contains water enough to deactivate a catalyst in all of the above processes, when it is recycled as a raw material for an ester exchange reaction or esterification reaction, water contacts the catalyst and affects the activity thereof, thereby making the fluctuation of the process inevitable.
JP-A 04-65426 discloses a polyester production process in which a distilling glycol component is directly recycled in limits to ensure that the total content of water contained in all the raw materials should be 0.5 wt % or less. In this case, it is possible to stabilize the process but the glycol component which can be recycled is very small in quantity and most of the distilling glycol component must be purified. Therefore, the installation of distillation equipment is essential and it cannot be said that this process is economical.
JP-B 7-100734 shows a polybutylene terephthalate direct polymerization method in which a glycol component containing tetrahydrofuran and water which is a condensate from a polycondensation device is supplied to a distillation column attached to an esterification reactor in examples. This polyester production method is effective because new distillation equipment does not need to be installed as a subsidiary equipment. However, as tetrahydrofuran and other low-boiling substances are supplied at the same time, the distillation load of the distillation column attached to the ester exchange reactor or esterification reactor increases. This is not taken into account in this method and it is difficult to achieve the target total content of low-boiling substances by this method alone.
JP-A 53-126096 teaches a process for producing polyethylene terephthalate by the direct polymerization method in which an ethylene glycol component formed in an esterification reaction step is introduced into a distillation column attached to an esterification reactor to remove low-boiling components and then recycled as a raw material. Only the esterification reaction step is taken into account in this process and the recycling of the ethylene glycol component formed from the polycondensation step as it is is not mentioned at all. That is, ethylene glycol formed in the polycondensation step must be purified in a large-scale distillation step separate from a system. Therefore, it cannot always be said that this process is economical.
JP-A 60-163918 discloses a process for producing polyethylene terephthalate by the direct polymerization method in which a gas essentially composed of an ethylene glycol component formed from a polycondensation reactor is condensed by a wet type condenser and the condensate is introduced into a distillation column attached to an esterification reactor to remove low-boiling impurities and recycled as a raw material. This polyethylene terephthalate production process is effective because new distillation equipment does not need to be installed as a subsidiary equipment but the distillation load of the distillation column attached to the esterification reactor increases and a large distillation column must be installed. Therefore, it cannot always be said that this process is economical and it is difficult to achieve the target total content of low-boiling substances by this process alone.
JP-A 55-56120 discloses a process for producing polyethylene terephthalate by the direct polymerization method in which an ethylene glycol component distilling out from an esterification reaction step and an ethylene glycol component distilling out and recovered from a polycondensation reaction step are mixed together and recycled as a raw material. In this process, ethylene glycol which is purified in a separate step must be supplied to a wet type condenser attached to a polycondensation reactor as a recycled liquid in large quantities. Like other processes, ethylene glycol must be purified with distillation equipment in a separate step. Therefore, it cannot always be said that this process is economical.
In view of the above problems, it is an object of the present invention to provide a process for continuously producing a polyester by polymerizing an aromatic dicarboxylic acid or a lower alkyl ester thereof and a glycol component essentially composed of an alkylene glycol having 2 to 4 carbon atoms, wherein the glycol component essentially composed of an alkylene glycol having 2 to 4 carbon atoms distilling out from a polycondensation reaction step is recycled as a raw material without purification using large-scale distillation equipment, the reaction step does not become instable thereby, a polyester having stable quality is obtained, the equipment can be simplified, and the running cost can be cut.
Other objects and advantages of the present invention will become apparent from the following description.
According to the present invention, the above objects and advantages of the present invention are attained by a process for continuously producing an aromatic polyester comprising an aromatic dicarboxylic acid as the main dicarboxylic acid component and at least one glycol selected from the group consisting of ethylene glycol, 1,3-propanediol and 1,4-butanediol as the main glycol component through an esterification or ester exchange reaction and a polycondensation reaction, wherein the distillate containing the above glycol from the polycondensation reaction is subjected to at least flush distillation to remove low-boiling substances and at least part of the residue is recycled to the above esterification or ester exchange reaction as part of the above glycol.